Soil is a relatively thin layer of unconsolidated material formed on the earth's surface, which develops from the interactions of air, water, plants and animals with parent geological material such as consolidated rock or material that has been deposited by wind, water, or ice. It typically takes thousands of years for centimeters of topsoil to form. This is because weathering or the physical breakdown of rocks into progressively smaller pieces, the transformation of primary minerals into secondary ones, and the churning of soil by plants and animals are all very slow processes under natural conditions, especially in temperate regions.
Native undisturbed soil typically displays a vertical sequence of layers differing in appearance and physical properties. The uppermost mineral layer is commonly referred to as topsoil. It corresponds to the plow layer of cultivated soils, or the A-horizon of undisturbed native soils. The thickness of a plow layer is typically 15 to 20 cm, but that of an undisturbed A-horizon layer may vary widely (e.g. up to 60 cm). The A horizon is characteristically enriched in organic matter and darker-colored than the underlying soil. By convention (e.g. in Canada), it must contain less than 17% w/w organic carbon or it would be described as organic soil.
The layer below the A-horizon is the B-horizon. The B-horizon is recognized by the accumulation of clay and humus having come from the A horizon. The depth of the B horizon depends on the intensity and duration of soil development. It can be absent in young soils. The C-horizon is the layer of unconsolidated material below the A- or B-horizon. It is the layer of undifferentiated unconsolidated material sitting on top of the consolidated rock or parent geological material. The layers below the A-horizon are generally referred to as the subsoil.
Generally, subsoil does not have the chemical (i.e., fertility) and physical (i.e., tilth) characteristics to sustain high plant productivity over time. It is often very difficult to treat subsoil such that it acquires and retains the same fertility as natural topsoil. Even when copious amounts of organic amendments, such as manure, are added to subsoil to increase its organic carbon content, amended subsoil rarely behaves like natural topsoil. There are several possible reasons for this, including that organic carbon in manure, straw or compost differs widely from naturally aged organic carbon (i.e. humus) formed in topsoil in terms of stability and susceptibility to microbial decomposition. Most commonly used organic amendments are readily degradable by microbial soil decomposers compared to humus and undergo rapid decomposition in soil. As a result, only a minute fraction of carbon added in the form of readily decomposable organic amendments typically ends up making the conversion into more recalcitrant humus in the soil.
Current regulatory requirements (e.g., in Canada) for the closure of oil and gas well sites require returning the sites to pre-disturbance agricultural productivity levels. This reclamation typically requires importing natural topsoil to spread onto the site, and demonstrating equivalent plant productivity of the reclaimed land. Prior to the mid-1970s, there were no land reclamation regulations and little incentive to conserve stripped native topsoil for later re-use in site reclamation. As a result, stripped native topsoil was generally hauled away to nearby farmlands where it can no longer be found or retrieved. Stripping topsoil during well site construction is considered necessary to improve the mechanical foundation quality of the soil and to facilitate vegetation control and minimize the risk of brush fires. Many of the well sites built in the early 1970s and earlier have reached the end of their useful operational life and now require abandonment and reclamation. Few have access to stockpiles of salvaged native topsoil. As a result, most of these sites now require that topsoil be imported from other locations to meet current land reclamation standards. The availability for purchase of topsoil of acceptable quality tends to vary as a function of the frequency and magnitude of large-scale land development projects in the vicinity of the site. In remote areas and rural regions where land development projects are unusual, the importation of native topsoil from distant construction sites is often very difficult and expensive.
In these remote and rural areas and elsewhere when topsoil is not readily available, there is a need to develop engineered topsoil in situ.
Others have attempted to produce artificial topsoil from recycled materials. U.S. Pat. No. 5,472,475 to Adam discloses the production of artificial topsoil by combining either dredged river silt, sand or basalt, and cellulose from recycled paper or yard waste, with composted animal or human waste for general reclamation and cereal crops, or vegetable or fruit residuals for gardens or potting soil. Adam further discloses the addition of calcium in the form of calcium silicate as slag or lime and ammonium nitrate or sulfate mixed with water. Charcoal or an equivalent amount of phosphorus, sodium, and sulfur as are present in charcoal is then added in small amounts. The process disclosed by Adam is not particularly well suited for in situ topsoil engineering nor is it appropriate for use in jurisdictions where the success of land reclamation must be demonstrated on the basis of prescribed measurements designed to show that the engineered topsoil has equivalent chemical composition and productive capability as that of the native surrounding topsoil at any given location.
A rate of soil genesis of one cm per year is considered exceptionally rapid in most world locations. In some regions (e.g., Canada), one mm per year can be expected. There is a need to increase the rate of topsoil formation. There is a need to engineer topsoil at a much higher than natural rate of soil formation.